1. Field of the Invention
The present invention relates to a magnetic incremental encoder, and in particular, relates to a magnetic incremental encoder suitable for a survey instrument, such as a total station or theodolite.
2. Description of the Related Art
In a known survey instrument, such as a total station or theodolite, a magnetic incremental encoder is provided as an angle measuring device. The magnetic incremental encoder is provided with a magnetic drum which rotates together with a collimating telescope, and a magnetic sensor which detects the angular displacement of the magnetic drum. The magnetic drum is provided on its outer peripheral surface with a multipolar magnetization layer which is divided into T magnetized sections (T is a positive integer) arranged at an equal pitch. The magnetic sensor is opposed to the multipolar magnetization layer. The magnetic sensor is provided with, for example, four magnetroresistor elements which are spaced at a distance smaller than the pitch of the magnetized sections of the multipolar magnetization layer. Consequently, the angular displacement of the magnetic drum is detected with an accuracy depending on the pitch of the magnetized sections, by detecting the reluctance of the magnetroresistor elements which varies in accordance with the rotation of the magnetic drum. An angle smaller than that corresponding to one pitch is determined by interpolation calculation.
For a survey instrument, the deviation of a protractor is specified by JIS (Japanese Industrial Standard) regulations, or the like. Accordingly, in a high precision survey instrument, two magnetic sensors are diametrically opposed to each other by 180 degrees, so that an arithmetic mean of the detection values of the two sensors is obtained to correct the eccentricity.
However, in case of a magnetic incremental encoder, the dividing number of the magnetized sections of the multipolar magnetization layer is smaller than that of an optical encoder, thus resulting in an increased pitch of the magnetized sections, and hence an influence of a harmonic error (harmonic distortion) within one pitch of the split sections is enhanced due to a dimensional error or deviation of magnetroresistor curves of the magnetroresistor elements from an ideal curve.
It is an object of the present invention to provide a magnetic incremental encoder suitable for a survey instrument, in which a plurality of harmonic distortions of an nth order (n is an optional integer) can be corrected simultaneously.
To achieve the object mentioned above, according to an aspect of the present invention, a magnetic incremental encoder including a plurality of detection devices which are provided on an outer periphery of a rotational member at a predetermined phase difference, wherein each of the detection devices outputs signals which vary periodically in accordance with the rotational angle of the rotational member upon rotation thereof; wherein the plurality of detection devices are provided in sets of two detection devices on the outer periphery of the rotational member, wherein a phase difference between one of the sets of two detection devices and an adjacent another of the sets of two detection devices is determined according to the following formula:
2xcfx80P+xcfx80/j; wherein P designates an arbitrary integer; and j designates one of the number of k orders, wherein k designates the number of orders of harmonic distortions to be corrected. The number of the plurality of detection devices is equal to 2k.
Preferably, the magnetic incremental encoder further includes a calculation device which calculates an arithmetic mean of the detection signals of the plurality of detection devices to obtain a detection angle whose harmonic distortions of k order have been corrected.
In order to correct k number of different order harmonic distortions, the phase difference between one of the sets of two detection devices and an adjacent another of the sets of two detection devices is determined by the above formula at a first order of j order of the number of k orders, wherein the one of the sets of two detection devices and the adjacent another of the sets of two detection devices constitute a group of detection devices.
Furthermore, the phase difference between one group of detection devices and an adjacent another group of detection devices of the plurality of detection devices is obtained by the formula at a j order other than said first order, of the number of k orders.
In order to correct nth order and mth order harmonic distortions, two sets of detection devices are provided; each set of detection devices including two detection devices, each detection device of the two sets of detection devices being arranged with a phase difference determined by one of the following formulae:
2xcfx80P1+xcfx80/n; and
2xcfx80P2+xcfx80/n;
wherein P1 and P2 are arbitrary integers. Each set of detection devices are arranged with a phase difference determined by the following formula:
2xcfx80P3+xcfx80/m;
wherein P3 is an arbitrary integer.
The rotary member is rotatably supported by a stationary portion of a device on which the magnetic incremental encoder is mounted, and the rotary member being provided on the outer peripheral surface thereof with a multipolar magnetization layer having split sections magnetized at an equal pitch of xcex; the detection devices being provided with magnetic sensors mounted to the stationary portion and opposed to the multipolar magnetization layer of the magnetic drum.
The magnetic sensors are each provided with eight magnetroresistor elements arranged with a phase difference of xcex/4 to detect the magnetic field which varies in accordance with the rotation of the magnetic drum. A detection signal is produced in accordance with the rotation angle of the magnetic drum, based on the detection outputs of four magnetroresistor elements which are deviated by xcex/2 and the detection outputs of the remaining four magnetroresistor elements which are deviated by xcex/2.
The calculation device can calculate the arithmetic mean of the rotation angles detected by all the magnetic sensors to obtain a rotation angle of the magnetic drum.
It is possible to provide additional detection devices, which have the same arrangement and number as the plurality of detection devices, are provided diametrically and symmetrically opposed to the plurality of detection devices, with respect to the center of rotation of the rotational member.
According to another aspect of the present invention, a survey instrument is provided having a magnetic incremental encoder including a plurality of detection devices which are provided on an outer periphery of a rotational member at a predetermined phase difference, wherein each of the detection devices outputs signals which vary periodically in accordance with the rotational angle of the rotational member upon rotation thereof; wherein the plurality of detection devices are provided in sets of two detection devices on the outer periphery of the rotational member, wherein a phase difference between one of the sets of two detection devices and an adjacent another of the sets of two detection devices is determined according to the following formula:
2xcfx80P+xcfx80/j; wherein P designates an arbitrary integer; and j designates one of the number of k orders, wherein k designates the number of orders of harmonic distortions to be corrected. The number of the plurality of detection devices is equal to 2k. The magnetic incremental encoder constitutes at least one of a vertical and horizontal protractor.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-178306 (filed on Jun. 14, 2000) which is expressly incorporated herein by reference in its entirety.